Ammonium nitrate propellant compositions containing thermoplastic polymeric resins



United States Patent O 5 Claims ABSTRACT OF THE DISCLOSURE Novelpropellant compositions comprising ammonium nitrate as a primaryoxidizer and thermoplastic polymeric resins having ionic and covalentbonds, as the binder.

BACKGROUND OF THE INVENTION This invention relates to a new class ofpropellant compositions and more particularly to ammonium nitratecompositions having improved ballistic properties, as a result of anovel binder system.

The use of ammonium nitrate based compositions as a solid propellant isattractive because of the cheapness and availability of ammoniumnitrate, because of the relatively low flame temperature 'ofdecomposition of ammonium nitrate, and because the excess free oxygenavailable from the decomposition permits the use of oxidizable materialto improve the energy available from the decomposition. However, thephysical characteristics of ammonium nitrate and grain material producedtherefrom introduced problems with respect to choice of binder material.Solid ammonium nitrate exists in different crystalline forms atdifferent temperatures and the transition from one form to a differentform involves a volume change of the ammonium nitrate. Volume changeswhich occur at about 90 F. and also at about 0 F. are particularlydestructive to shaped propellant grains. It is, therefore, obvious thatammonium nitratebased compositions could be seriously affected bystorage at temperatures common to such storage conditions.

One requirement for solid propellants suitable for military use in thatthe propellant be ballistically stable after long periods of storage attemperatures as high as 160 F. .and as low as -65 F. Another requirementis that the grain composition not shatter or crack after being subjectedto alternate high and low temperatures (i.e. cycled from the hightemperature immediately to the low temperature at least twice); and thatthe burning of the composition following such cycling be uniform and notchanged materially from the burning characteristics of such grainmaterial which has not been subjected to such cycling. The bindermaterial used with the ammonium nitrate to form physically stable grainsmust be flexible enough to compensate for changes in volume of theammonium nitrate as it passes from one temperature to another, in orderthat such changes produce a minimum amount of voids and cracks in thegrain. Production of fissures in the grain either internally orexternally of the surface of the grain creates additional burningsurface which results in unpredictability of the ballistic performanceof the grain. Furthermore, the binder material must be of such nature topermit grain formation by methods known to the art, i.e. extrusion,molding, etc.

SUMMARY OF THE INVENTION I have now discovered a novel propellantcomposition comprising ammonium nitrate as the oxidizer and athermoplastic resin having ionic and covalent bonds, as the binder. Theoxidizer and binder must be in intimate ice physical mixture. Thepresent compositions may also contain various other additives such ascatalyst for the promotion of combustion, carbon, chemical stabilizer,etc.

The term ammonium nitrate as used in this specification and in theclaims is intended to mean either ordinary commercial grade ammoniumnitrate such as conventionally grained ammonium nitrate containing asmall amount of impurities which may be coated with a small amount ofmoisture-resisting material such as petrolatum or parflin, or to meanmilitary grade ammonium nitrate or mixture of minor amounts (usuallyless than 10%) of other organic or inorganic nitrates such as, guanidinenitrate or sodium nitrate or potassium nitrate with the ammoniumnitrate. A mixture of finely ground and unground or coarsely groundammonium nitrate is preferred. The major proportion of the ammoniumnitrate should be finely ground in order to reduce the voids to aminimum and hence avoid the use of excess binder material. It is wellknown in the propellant art that particle size and size distributionWithin the composition are related to the ballistic properties. Theamount of grinding then, will depend upon the desired properties of thepropellant.

The polymeric binder material is a cross-linked thermoplastic resinlinked by ionic and covalent bonds, characterized by strong interchainforces. These interchain forces are developed between ionized carboxylgroups pendant from an alpha olefin chain and metallic ions selectedfrom the group consisting of sodium, potas sium, calcium, magnesium andzinc. These products may be obtained commercially from E. I. du Pont deNemours and Company under the name Surlyn A. The major constituent ofSurlyn A is ethylene. The polymeric binder has a softening point in therange of to 120 C., and a preferred softening point of to C. These newpropellant compositions have the advantage of excellent ballisticproperties, simplicity of formulation and stability. Normal mixing ofthe composition is accomplished without the necessity of additionalplasticizers. The present compositions provide a propellant which may beutilized Where a low burn rate is required.

It is desirable that catalyst be present in the composition and anycatalyst known to be effective for the combustion of ammonium nitrategrains containing oxidizable thermoplastic binder materials may be used.These include certain iron compounds broadly designed as Prussian Blues,sodium barbiturate, etc. Catalysts which are exceptionally suitable tothe present compositions are disclosed in copending application, S.N.655,328, filed July 24, 1967. These compositions comprise hydrocarbonswhich readily form peroxides.

It is also desired that the composition contain a carbon component. Thefinely divided carbon may be added in amounts up to 10% by'weight of thecomposition, preferably in amounts of l to 5% by weight for the purposeof improving ignition and the burning rate of the composition. Highlyadsorptive activated carbons such as Norite and Nuchar Well known in theart as activated carbon of vegetable origin, make up one class ofeffective burning rate components. A second general class of carbonuseful for increasing the burning rate of compositions are the carbonblacks, roughly classified as the channel blacks and the furnacecombustion blacks. The carbon blacks are characterized by low ashcontent, that is, less than about 0.5%, usually less than about 0.15%,and by having extremely small particle size, that is 50 to 5,000 A. andcontain adsorbed hydrogen and oxygen. Bead type carbon blacks, such asMicronex Beads and Statex Beads, are also suitable. A third type ofcarbon which may be used is finely ground petroleum coke, particularlypetroleum coke obtained as residue in the pipe-stilling of mid-continentheavy residuums.

The oxidizer and binder should be present in the composition inessentially stoichiometric amounts, in order to best utilize the excessoxygen obtained from the oxidizer. Therefore, a major amount of oxidizerand a minor amount of binder should be present. Suitable propellantcompositions comprise:

Percent by weight (a) Binder to 40 (b) Catalyst 0.5 to 5 (c) Carbon Oto(d) Remainder of composition essentially ammonium nitrate.

Preferred compostions are as follows:

Percent by weight (a) Binder to (b) Catalyst 1.5 to 3.5 (0) Carbon 1 to5 (d) Remainder of composition essentially ammonium nitrate.

Other additives which may be added are present in an amount sufficientto provide effective improvement of the properties for which they areadded.

In preparing the composition of this invention, any procedure known tothe art for the separation of ammonium nitrate grains containing athermoplastic binder may be used.

With respect to the properties of a solid propellant, the velocity atwhich a solid propellant is consumed during operation is called theburning rate. It is measured in a direction normal to the propellantsurface and is usually expresed in inches per second. The burning ratemay be expressed by the following relation, in which the influence ofall performance parameters is small compared to the chamber pressure andthe initial grain temperature.

The burning rate or velocity of propellant consumption r is usuallygiven in inches per second; the chamber pressure p in pounds per squareinch; a and n are constants. The constant a varies with the initialpropellant temperature, and thus the burning rate is a function of thetemperature of the grain prior to combustion. The lower the value of n,the less is the detonating character of the decomposition of a gasproducing composition and the more even and smooth is the burning rateof the propellant grain, Thus, a sustained thrust rather than adetonation is obtained by smooth burning of the grain.

The temperature sensitivity for different solid propellants is usuallyexpressed as the percentage change of thrust per unit of temperaturechange. Temperature changes effect the equilibrium pressure and theburning rate. The definitions of the temperature coefficients are givenby Sutton, Rocket Propulsion Elements (2nd ed.

111 P) i (E T K P 6T K 6lnr l (61) 5T r 6T D Here Hg is the temperaturesensitivity coefficient 0i equilibrium pressure at a particular value ofK (K is the ratio of the burning surface to the throat area), expressedin percent pressure change per degree temperature change. Mathematicallyit is defined as the partial derivative of the natural logarithm of theequilibrium chamber pressure p with respect to temperature T. The othertemperature sensitivity coefficient u refers to the change in burningrate r of a solid propellant with respect to temperature T at aparticular value of chamber pressure p It is also known as the burningrate tempertaure coefiicient, while nis known as the of-pressure... V i

For most propellant applications, as low a temperature coefficient aspossible is desirable and even required for engineering designconsideration. Lower pressure levels over a given temperature levelallows a sizable weight savings for most missile applications,

DESCRIPTION OF PREFERRED EMBODIMENTS The following examples are given byway of illustration and should not be construed as limiting.

EXAMPLES I v u temperature sensitivity Weight Example FormulationPercent Catalyst Type I Ammonium nitrate 78.0

2: Octahydroanthracene. Catalyst; 2. II Ammonium] 78.

' Plieuylcumene.

Silica-alumina.

Cumene.

}Tetralin and cumene.

Catalyst Ammonium nit VII }Asphalt.

The above formulations were molded into grains and cut into strands fortesting in a Crawford bomb. The results of the tests are shown in TableI.

TABLE I Ballistic Properties Percent F. Product of T1000 Example 02 Up'nK (in/see.)

Table I clearly shows excellent ballistic properties afforded by thepresent composition.

What is claimed is:

1. A propellant composition comprising:

(a) a major amount of ammonium nitrate in intimate physical mixturewith;

(b) a polymeric binder material, having a softening point in the rangeof. from 80 C. to C., said binder material comprising a cross-linkedthermoplastic resin containing:

(i) a major amount of ethylene units; and

(ii) interchain forces developed between carboxyl groups pendant from analpha olefin chain and metallic ions selected from the group consistingof sodium, potassium, calcium, magnesium, and zinc; and

(c) an effective amount of combustion catalyst.

2. The composition of claim 1 wherein said polymeric binder material ispresent in an amount from 5 to 40 weight percent; said catalyst ispresent in an amount from 0.5 to 5 weight percent; carbon is present inan amount from 0 to 10 weight percent; and the remainder of saidcomposition is essentially ammonium nitrate.

3. The composition of claim 2 wherein said polymeric binder material ispresent in an amount of from 15 to 30 weight percent; said catalyst ispresent in an amount of from 1.5 to 3.5 weight percent; said carbon ispresent in an amount of from 1 to 5 percent; and the remainder of saidcomposition is essentially ammonium nitrate.

4. The composition of claim 1 wherein said polymeric References CitedUNITED STATES PATENTS 9/1964 A-bere et a1. 14919 2/1967 Burnside 14919 X8/1967 Evans 149-19 BENJAMIN R. PADGE'IT, Primary Examiner U.S. C1.X.R.

